In fluidized reaction systems for paraffin and/or alkyl aromatic dehydrogenation to the associated olefin, the thermal (gas phase) reaction of the paraffin and/or alkyl aromatic feedstock is sometimes significantly lower in selectivity than the catalytic selectivity. For example, with propane the thermal dehydrogenation to propene selectivity is about 45 to 50 mol % while the catalytic dehydrogenation to propene selectivity is about 99 mol % or greater. Likewise, ethylbenzene thermal dehydrogenation to styrene selectivity is about 67 mol % while ethylbenzene catalytic dehydrogenation selectivity is about 99 mol % or greater.
Up-flow fluidized reactors are economical means to dehydrogenate alkanes and alkyl aromatics. Specifically, risers, turbulent bed reactors, bubbling bed reactors, or fast fluidized reactors have the advantage of being able to carry out the dehydrogenation reaction at minimal residence times. However, transport of the product gas and solids to a catalyst separation system and the separation system itself increases overall gas residence time. This additional gas residence time results in less selective reaction of the feedstock resulting in a lower overall reactor selectivity to the desired product.